Hydrogen sulfide (H2S) is a lethal gas regarded as a potential chemical threat by the Department of Homeland Security and a chemical hazard immediately dangerous to life by the Centers for Disease Control. We have recently developed a paradigm to decrease H2S very rapidly in the blood and tissues of rats based on the use of the crystalline form of low dose of methemoglobin (as a source of ferric Iron) and high dose of Vitamin B12 (as a source of oxidized Cobalt). Both agents are safe (high dose of Vitamin B12 is already FDA approved for cyanide poisoning), can be stored as a kit, in a powder form, and can easily be administered to humans following H2S exposure. The goal of this project is to test the hypothesis that solutions of methemoglobin or Vitamin B12 can be used as antidotes following H2S exposure. The possibility of administering Vitamin B12 in H2S intoxication at a dose 10 times lower than the dose used for cyanide intoxication makes it possible to deliver this agent using intramuscular or intranasal administration. Since we have demonstrated that H2S produces in small animals (rodents) a specific protective response, not found in larger mammals, we intend to conduct our studies in adult sheep. In this proposal, we will determine the ability of these antidotes to lower the concentration of H2S, and its oxidative products, in th blood and the cerebrospinal fluid of adult sheep following infusion of lethal concentrations of H2S. The cerebrospinal fluid will be used as a convenient marker of the exposure of the medullary neurons, exquisitely sensitive to the toxic effects of H2S. At the same time, we will assess the alterations of the respiratory and cardiovascular control systems. H2S intoxication will be generated by intravenous infusion, prepared from a solution of NaHS, a H2S donor, to prevent any risk to the laboratory personnel related to gaseous H2S exposure. H2S infusion will be stopped as soon as an apnea occurs and at this time the end-points proposed in this application will be evaluated in controls and in animals treated with either methemoglobin or Vitamin B12 given one or 20 minutes following the cessation of H2S infusion. Finally, we will determine the blood concentration of Vitamin B12 and the ability of the plasma to oxidize exogenous H2S, following intranasal or intramuscular administration of Vitamin B12. The results of these proposed studies will help determine the proper strategy of use and the respective benefits of these antidotes. Intranasal or intramuscular administration of Vitamin B12 represents an interesting alternative to intravenous infusion that could be implemented by non-professional rescuers to a large number of patients.